Where is the function to address an instrument to talk?

Hi all, my question might look stupid, but I am curious

to figure out how labview does to control a instrument , in a systematic view!

I read the operational manual for HP8903B,  it says that it send the data message when addressed to talk,

So I believe we can only read the measurement result when this instrument is addressed to talk.

I downloaded the "hp8903b driver for Labview2012", and looked through some application examples.

and found the HP8903B Initialize.vi inside, where there is a VISA open function in it,

I read the file "visa.h", but did not find , how labview did to address an instrument to talk,

I think it has called some lower level functions,

where can I find them?